In recent years, much activity and invention in the art of polyesters has centered around the development of articles made from polyethylene terephthalate or PET. Containers constructed of PET have recently found wide acceptance in the packaging of food stuffs, medicines and consumer products.
The manufacture of PET and various copolymers has heretofore generally required the use of catalysts based on heavy metals such as antimony, manganese, titanium, etc. If residues of these catalysts remain in the polyester material, the clarity of the material is greatly reduced such that the material exhibits a hazy appearance. Further, the residues act as nucleating agents for crystallizable polyesters such as PET thus making the control of crystallization more difficult, for example, the temperature of the reaction must be kept within very narrow limits to prevent excessive crystallization which is also manifested as haze. Therefore, for many applications, removal of the catalyst residue is required after polymerization, commonly by filtration or metal complexation. These additional steps of course add considerably to the processing costs and are in any case never completely successful in residue removal. In addition, metallic catalyst residues can also accelerate the formation of undesirable by-products such as acetaldehyde during the preparation and/or processing of the polyester material.
Reference to the use of acids as polymerization catalysts can be found in Organic Chemistry of Synthetic High Polymers, Lenz, R. W., John Wiley, New York, N.Y., 1967, pages 81-91, and Textbook of Polymer Science, 2nd Ed., Billmeyer, F. W., Wiley-Interscience, New York, N.Y., 1971, pages 264-267. While the authors generally describe the use of acids, they do not disclose the use of protonic acid catalysts to increase clarity in crystallizable polyesters.